Mass spectrometric decomposition processes of 2-methyl-1-hexene

Unimolecular decompositions of 2-methyl-1-hexene and several labelled analogues were studied following 70 eV electron impact (normal and metastable spectra) and field ionization (field ionization kinetic measurements). Molecules labelled with ¹³C in the 1-position and the methyl position were found to behave essentially identically. This is attributed to rapid transfer of a hydrogen atom mainly from C-5 to C-1 (γ-hydrogen shift). Loss of ethene, propene or propenyl do not involve loss of the methyl carbon or C-1. All three reactions are better than 90% specific in this respect under all conditions studied. At shorter times, C₃H₆ loss is the dominant reaction, while at longer times C₂H₆ loss accounts for >90% of the ion current. It is proposed that at least two distinct pathways for C₂H₄ loss operate in linear 1-alkenes, one of which (loss of carbons 1 and 2) is blocked by a 2-methyl substituent. The C₆H₁₁]^+˙ and C₅H₁₀]^+˙ ions formed from ¹³C labelled 2-methyl-1-hexenes fragment in an essentially statistical fashion.